There are situations where high intensity light beams with restricted aperture geometry are desirable, but connecting a conventional incandescent lamp/reflector assembly having a low aspect ratio (i.e., the width of the aperture is comparable to the height) to a narrow slit having a high aspect ratio (i.e., the width of the aperture is large relative to the height) without significant optical losses has been difficult to achieve while maintaining farfield intensity. For example, the circular lamp lens can be taped or painted to create a narrow slit, but in doing so, most of the light energy would be lost. Conventional tapered or fiber optic light guides produce divergent output beams. The present invention is a simple and practical light guide to efficiently couple light from a low aspect ratio source, like a circular sealed beam lamp, to a high aspect ratio (narrow slit) aperture.
Obtaining optimum optical efficiency in such coupling has plagued other prior art solutions. Fiber optic light guides, bundles of fibers or small diameter transparent rods functioning as light guiding fibers, internally reflective tubes, or various configurations of bent and twisted transparent slabs all offer generally poor optical efficiency because they cause the output to spread or to diverge. These prior art solutions rely upon a multitude of internal reflections. Each reflection, however, alters the eventual emergent angle of the reflected ray so that a collimated input will be converted to a widely divergent output. This output would provide reduced levels of energy/unit area in the far field. Such a light would be a poor headlight because of its short range. An example of this type of light is shown in U.S. Pat. No. 2,704,321. The result is not a narrow beam of light with high intensity in the far field.
U.S. Pat. No. 5,276,592 discloses a high aspect ratio source that relies on a plurality of lenses and mirrors to distribute light from a small light source to a narrow, forward-directed aperture. Such a lens system is expensive to manufacture and difficult to align. It relies on an internal, small source rather than accepting a large diameter beam from a separate source. The present invention provides a unitary, plastic light guide to produce the desired conversion of an external light source to a high aspect ratio aperture.